1. Field of the Invention
The present invention relates to an optical scanning device, optical scanning method, and image forming apparatus.
2. Discussion of the Background
There has been proposed an optical scanning method which divides an area to be scanned on a surface to be scanned into n (wherein nxe2x89xa72) partial scanning portions in the longitudinal direction and uses n light sources corresponding to the partial scanning portions on a one-to-one basis to optically scan the partial scanning portions corresponding to the light fluxes from the n light sources, thereby synthetically optically scanning the area to be scanned (Japanese Unexamined Patent Application Publication No. 7-199098, Japanese Unexamined Patent Application Publication No. 10-213763, etc.).
Such an optical scanning method (hereafter referred to as xe2x80x9csegmented scanningxe2x80x9d) has the following advantages.
First, increasing the number n of partial scanning portions constituting the area to be scanned allows optical scanning to be performed over extremely wide scanning areas.
Second, each partial scanning portion can be made small, so that in the event of individually providing plural scan imaging optical systems for each partial scanning portion, for example, the angle of each scan imaging optical system does not have to be made unnaturally wide, and the plural scan imaging optical systems and the optical elements constituting the plural scan imaging optical systems can be made small. Further, correction of wave-front aberrations which are correlated with the light spot diameter can be facilitated, and there is less difference in spot diameter due to irregularities in parts and attachment variances, so that the diameter of the light spot can be reduced.
However, on the other hand, dividing the area to be scanned into multiple partial scanning portions results in the optical scanning of the partial scanning portions by separate light fluxes, and in the event that the intensity of light, spot diameter, optical scanning speed, etc., of the light spots irradiating the surfaces to be scanned differ from one partial scanning portion to the next, the latent image written by optical scanning upon developing will show irregularities in concentration as the image density differs from one partial scanning portion to another, e.g., the concentration of band-shaped portions extending in the sub-scanning direction, corresponding to the partial scanning portions, can change cyclically according to the length of the partial scanning portions in the main scanning direction.
A non-uniformity in the intensity of light irradiated on the surface to be scanned from one partial scanning portion to the next can occur due to variances the emitted light intensity between different light sources and the properties of transmittance and reflectance of the optical elements situated between the different light sources and the partial scanning portions.
Also, a non-uniformity in the spot diameter from one partial scanning portion to another can occur due to manufacturing variances in the individual optical elements situated between the different light sources and the partial scanning portions, variances in the precise attachment thereof, and so forth.
Further, a non-uniformity in the optical scanning speed from one partial scanning portion to another can occur due to manufacturing variances in the fxcex8 lens used as the scan imaging optical system, and variances in the precision of attachment thereof.
When there is a difference in image density at image portions corresponding to adjacent partial scanning portions, the image density or concentration changes in steps at the portion corresponding to a seam between the partial scanning portions, and accordingly irregularities in image density tend to be conspicuous.
With the invention described in Japanese Unexamined Patent Application Publication No. 10-213763, the arrangement is such that the amount of light flux optically scanned in each partial scanning portion is uniform near a seam between adjacent partial scanning portions. However, irregularities in concentration can still occur if the spot diameter or optical scanning speed differs from one partial scanning portion to another.
The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems.
Accordingly, preferred embodiments of the present invention provide a novel optical scanning device and a novel optical scanning method that effectively reduce the above-described image density or concentration irregularities in optical scanning performed by segmented scanning, and thereby make irregularities in concentration less conspicuous.
According to a preferred embodiment of the present invention, an optical scanning device divides an area to be scanned on a surface to be scanned into n (wherein nxe2x89xa72) partial scanning portions in the longitudinal direction, and uses n light sources corresponding to the n partial scanning portions on a one-to-one basis to optically scan the n partial scanning portions with respective light fluxes from the n light sources, thereby scanning optically the area to be scanned. The optical scanning device includes a deflecting device and a scan imaging optical device.
The n light sources are each independently capable of modulating the intensity of the emitted light. For example, n solid lasers or gas lasers, n LEDs, etc., may be used. Most practically, n semiconductor lasers may be used.
The n number of light sources is not restricted to those mentioned above. Each light source may have a plurality of light-emitting sources which are each independently capable of modulating the intensity of the emitted light, such as for example articles wherein light fluxes from multiple semiconductor lasers are synthesized with a light flux synthesizing prism, articles wherein light fluxes from multiple semiconductor lasers are mutually given angles and combined, semiconductor laser arrays, etc. Using such light sources for each partial scanning portion allows each partial scanning portion to be optically scanned with the multi-beam scanning method such that multiple lines in the area to be scanned are simultaneously synthetically scanned optically.
The deflecting device deflects the light fluxes from the n light sources, and an appropriate known deflector (e.g., a rotating unifacial mirror, a rotating bifacial mirror, a rotating polygonal mirror, a galvano-mirror, etc.) may be used.
One deflector may be provided for each of the n light sources so that the deflecting device is configured with a total of n deflectors. Alternatively, two or more light sources may share one deflector so that the deflecting device is configured from a number of deflectors which is less than the number n of light sources.
The scan imaging optical device is an optical device for converging each light flux deflected by the deflecting device toward the surface to be scanned, thereby forming a light spot at the partial scanning portion from each light flux.
For example, one set of image scanners (i.e. a scan imaging optical system) may be provided for each of the n light sources, thereby configuring the scan imaging optical device with n sets of image scanners. Alternatively, one set of image scanners which are configured to converge individual light flux from two or more light sources to the respective corresponding partial scanning portions may be utilized, thereby configuring the scan imaging optical device with a number of image scanners which is less than the number n of light sources. Accordingly in this alternate embodiment, when the number of light sources n=2, the scan imaging optical device is configured with only one image scanner.
Each of the image scanners may be configured with one or more lenses, or may be configured with one or more imaging reflectors (reflectors having imaging capabilities), or may be configured with a combination of one or more lenses and one or more imaging reflectors.
The xe2x80x9csurface to be scannedxe2x80x9d is essentially the photosensitive surface of the photo-electroconducting photosensitive member.
A partial scanning portion is each portion obtained by dividing the area to be scanned on the photosensitive face into n pieces in a longitudinal direction. Each partial scanning portion may have the same length, or may have mutually different lengths.
Also, each partial scanning portion may divide the same line in the main scanning direction into n parts, or, the partial scanning portions may be separated in the sub-scanning direction. When the partial scanning portions are separated in the sub-scanning direction, the timing for performing scanning of each partial scanning portion is offset, so that partial latent images formed on the n partial scanning portions connect one to another to form line in the latent image (multiple lines of the latent image in the event that the light source has multiple light sources).
The optical scanning device according to a first aspect of the present invention is arranged such that with the line width of one-dot lines, formed by scanning with the corresponding light flux near the center portion of the ith partial scanning portion of the n partial scanning portions, denoted as Li (wherein i=1 through n), the optical scanning condition is set so that the average value Ave(Li), the maximum value Max(Li), and the minimum value Min(Li), of the line widths Li of the one-dot lines satisfy the condition of:
Max(Li)/1.15 less than Ave(Li) less than Min(Li)/0.85.xe2x80x83xe2x80x83(1) 
The term xe2x80x9cone-dot linexe2x80x9d refers to a line-shaped latent image formed of a continuation of single dots.
The definition of the line width Li will be described later.
The average value Ave(Li) denotes the arithmetical mean of the line widths Li, i.e., (xcexa3Li)/n (wherein the sum is from 1 to n depending on i). This definition for Ave (Li) holds true in the following description, as well. The maximum value Max(Li) is the largest of the n number of line widths Li, and the minimum value Min(Li) is the smallest of the n number of line widths Li.
The optical scanning device according to a second aspect of the present invention is arranged such that with the line width and line concentration of one-dot lines formed by scanning with the corresponding light flux near the center portion of the ith partial scanning portion denoted as Li and Di respectively (wherein i=1 through n), the optical scanning condition is set so that the average value Ave{(Li)xc3x97Di{circumflex over ( )}(⅕)}, the maximum value Max{(Li)xc3x97Di{circumflex over ( )}(⅕)}, and the minimum value Min {(Li)xc3x97Di{circumflex over ( )}(⅕)}, of the computation value (Li)xc3x97Di{circumflex over ( )}(⅕) of the one-dot lines satisfy the condition of:
Max{(Li)xc3x97Di{circumflex over ( )}(⅕)}/1.15 less than Ave{(Li)xc3x97Di{circumflex over ( )}(⅕)} less than Min{(Li)xc3x97Di{circumflex over ( )}(⅕)}/0.85.xe2x80x83xe2x80x83(2) 
In the above expression, xe2x80x9cDi{circumflex over ( )}(⅕)xe2x80x9d is xe2x80x9cDi to the 1/5th power, i.e., xe2x80x9c{circumflex over ( )}xe2x80x9d indicates an exponent. This definition of Di{circumflex over ( )}(1/S) (1 holds true in all of the following description, as well.
The definition of the line concentration Di will be described later.
The average value Ave{(Li)xc3x97Di{circumflex over ( )}(⅕)} is the arithmetical mean of n number of computation values (Li)xc3x97Di{circumflex over ( )}(⅕), wherein i=1 through n, the maximum value Max{(Li)xc3x97Di{circumflex over ( )}(⅕)} is the greatest of the n computation values (Li)xc3x97Di{circumflex over ( )}(⅕), wherein i=1 through n, and the minimum value Min {(Li)xc3x97Di{circumflex over ( )}(⅕)} is the smallest of the n computation values (Li)xc3x97Di{circumflex over ( )}(⅕), wherein i=1 through n.
With the optical scanning device according to the second aspect, the average value Ave{(Li)xc3x97Di{circumflex over ( )}(⅕)}, the maximum value Max{(Li)xc3x97Di{circumflex over ( )}(⅕)}, and the minimum value Min {(Li)xc3x97Di{circumflex over ( )}(⅕)}, of the computation value (Li)xc3x97Di{circumflex over ( )}(⅕) more preferably satisfy the condition of:
Max{(Li)xc3x97Di{circumflex over ( )}(⅕)}/1.07 less than Ave{(Li)xc3x97Di{circumflex over ( )}(⅕)} less than Min{(Li)xc3x97Di{circumflex over ( )}(⅕)}/0.93.xe2x80x83xe2x80x83(2A) 
Here, near the center portion of the ith partial scanning portion, one-dot lines formed by scanning with the corresponding light flux may be one-dot lines which are long in the main scanning direction, one-dot lines which are long in the sub-scanning direction, or one-dot lines which are inclined in the main scanning direction.
In the event that the one-dot lines are one-dot lines which are elongated in the main scanning direction, and in the event that the light source has multiple light-emitting sources and the partial scanning portions are optically scanned by multi-beam scanning, the above conditions are satisfied by xe2x80x9ceach of the multiple one-dot linesxe2x80x9d formed by multi-beam scanning. This criterion holds true in all of the following description, as well.
With the optical scanning device according to the first aspect, near the center portion of the ith partial scanning portion, the condition (1) is preferably satisfied by the one-dot lines which are elongated in the main scanning direction and the one-dot lines which are elongated in the sub-scanning direction, formed by scanning with the corresponding light flux.
With the optical scanning device according to the second aspect, near the center portion of the ith partial scanning portion, the condition (2) is preferably satisfied by the one-dot lines which are elongated in the main scanning direction and the one-dot lines which are elongated in the sub-scanning direction, formed by scanning with the corresponding light flux, and further, near the center portion of the ith partial scanning portion, the condition (2A) is preferably satisfied by the one-dot lines which are elongated in the main scanning direction and the one-dot lines which are elongated in the sub-scanning direction, formed by scanning with the corresponding light flux.
The optical scanning device according to a third aspect of the present invention is arranged such that with a line width ratio Li/Li+1 (wherein i=1 through nxe2x88x921) of one-dot lines, formed by scanning with the corresponding light flux long in the sub-scanning direction near a seam between the ith partial scanning portion and the i+1th partial scanning portion, denoted as a sub-scanning line width ratio KS, the optical scanning condition is set such that the sub-scanning line width ratio KS satisfies the condition of:
0.9 less than KS less than 1.1.xe2x80x83xe2x80x83(3) 
The optical scanning device according to a fourth aspect of the present invention is arranged such that with a line width ratio Li/Li+1 (wherein i=1 through nxe2x88x921) of one-dot lines, formed by scanning with the corresponding light flux long in the main scanning direction near the seam between the ith partial scanning portion and the i+1th partial scanning portion, denoted as a main scanning line width ratio KM, the optical scanning condition is set such that the main scanning line width ratio KM satisfies the condition of:
0.93 less than KM less than 1.07.xe2x80x83xe2x80x83(4) 
In the third aspect of the present invention, with the line width ratio Li/Li+1 (wherein i=1 through nxe2x88x921) of one-dot lines, formed by scanning with the corresponding light flux long in the main scanning direction near the seam between the ith partial scanning portion and the i+1th partial scanning portion, denoted as the main scanning line width ratio KM, the optical scanning condition is preferably set such that the main scanning line width ratio KM satisfies the condition of:
0.93 less than KM less than 1.07.xe2x80x83xe2x80x83(4) 
The optical scanning device according to a fifth aspect of the present invention is arranged such that with the line width ratio Li/Li+1 (wherein i=1 through nxe2x88x921) of one-dot lines, formed by scanning with the corresponding light flux long in the sub-scanning direction near the seam between the ith partial scanning portion and the i+1th partial scanning portion, denoted as the sub-scanning line width ratio KS, and a line concentration ratio Di/Di+1 denoted as a line concentration ratio HS, the optical scanning condition is set such that the computation amount (KS)xc3x97HS{circumflex over ( )}(⅕) satisfies the condition of:
0.9 less than (KS)xc3x97HS{circumflex over ( )}(⅕) less than 1.1.xe2x80x83xe2x80x83(5) 
There are a total of nxe2x88x921 computation amounts (KS)xc3x97HS{circumflex over ( )}(⅕) since a computation amount is obtained near each seam of adjacent partial scanning portions, and these nxe2x88x921 computation amounts satisfy the condition (5).
The optical scanning device according to a sixth aspect of the present invention is arranged such that with the line width ratio Li/Li+1 (wherein i=1 through nxe2x88x921) of one-dot lines, formed by scanning with the corresponding light flux long in the main scanning direction near the seam between the ith partial scanning portion and the i+1th partial scanning portion as the main scanning line width ratio KM, and a line concentration ratio Di/Di+1, denoted as a line concentration ratio HM, the optical scanning condition is set such that the computation amount (KM)xc3x97HM{circumflex over ( )}(⅕) satisfies the condition of:
0.93 less than (KM)xc3x97HM{circumflex over ( )}(⅕) less than 1.07.xe2x80x83xe2x80x83(6) 
In the fifth aspect, with the line width ratio Li/Li+1 (wherein i=1 through nxe2x88x921) of one-dot lines formed by scanning with the corresponding light flux elongated in the main scanning direction near the seam between the ith partial scanning portion and the i+1th partial scanning portion, denoted as the main scanning line width ratio KM, and the line concentration ratio Di/Di+1 denoted as the line concentration ratio HM, the optical scanning condition is preferably set such that the computation amount (KM)xc3x97HM{circumflex over ( )}(⅕) satisfies the condition of:
0.93 less than (KM)xc3x97HM{circumflex over ( )}(⅕) less than 1.07.xe2x80x83xe2x80x83(6) 
With the fifth and sixth aspects, there are a total of nxe2x88x921 computation amounts (KM)xc3x97HM{circumflex over ( )}(⅕) since a computation amount is obtained near each seam of adjacent partial scanning portions, and these nxe2x88x921 computation amounts satisfy the condition (6).
In any of the above-described arrangements, a set light-emitting amount (i.e. an intensity) of the n light sources may be independently adjustable. The set light-emitting amount is the intensity in a reference light-emitting state which serves as a reference for intensity modulation, and can be set by a reference driving current value in the case of semiconductor lasers.
Arranging for the intensity of the light sources to be independently adjustable allows the above-described conditions (1) through (6) to be satisfied.
Also, in any of the above-described arrangements, a driving duty ratio (i.e., the ratio of light-emitting time of a light-emitting source relative to the writing time corresponding to one pixel) of the n light sources may be independently settable. Adjusting the driving duty ratio of the light sources allows the size of a dot image obtained by visualizing a dot formed by the light source to be adjusted, enabling independent adjustment of the driving duty ratio of each light source which allows the above-described conditions (1) through (6) to be satisfied. In this case, the adjustment of the driving duty ratio may be combined with the above-described adjustment of the light intensity.
Also, in any of the above-described arrangements, the openings of n number of apertures are configured to beam form each of the light fluxes from the n light sources independently.
The opening of each aperture defines a spot diameter, so that, by adjusting the opening of each of the apertures in both the main scanning and sub-scanning directions, independent adjustment of the size and shape of the light spot on the surface to be scanned is obtained in both the main scanning and sub-scanning directions. The opening of each aperture can be employed to satisfy the above-described conditions (1) through (6). In this case, adjusting the openings of the apertures may be combined with the adjustment of the duty ratio and/or the adjustment of the light intensity.
In any of the above-described arrangements, the number n of light sources may be two, and in this case, the deflecting device may be configured of two optical deflectors for individually deflecting each light flux from each light source. Here, the scan imaging optical device may be configured with two image scanners configured to individually converge light flux from each of the light sources toward the area to be scanned.
Also, in any of the above-described arrangements, the optical scanning device may include photo-receptors at the starting side and ending side of optical scanning device for performing photo-reception of deflected light flux, from each partial scanning portion. In this case, one photo-receptor may be placed between adjacent partial scanning portions, with the photo-receptor photo-receiving deflected light flux at an ending side of one partial scanning portion and also photo-receiving deflected light flux at a starting side of the other partial scanning portion. When each light source has multiple light-emitting sources and optical scanning of each partial scanning portion is carried out by multi-beam scanning, the arrangement may involve photo-reception of only one of the multiple light fluxes scanning the same partial scanning portion or all of the multiple light fluxes.
The optical scanning method according to the present invention divides an area to be scanned on a surface to be scanned into n (wherein nxe2x89xa72) partial scanning portions in the longitudinal direction and uses n light sources corresponding to the n partial scanning portions on a one-to-one basis to optically scan the partial scanning portions with corresponding light fluxes from the n light sources, thereby synthetically optically scanning the area to be scanned. The optical scanning method may be carried out using the optical scanning device according to the present invention, in any of the arrangements described above.
The optical scanning method may be carried out using the optical scanning device including photo-receptors at the starting side and ending side of optical scanning device for photo-receiving deflected light fluxes, at each partial scanning portion. Correspondingly, a driving clock for each light source corresponding to each partial scanning portion is adjusted based on a difference in detection time.
The image forming apparatus according to the present invention forms an electrostatic latent image by optically scanning a photo-electroconducting photosensitive member with an optical scanning device, and developing the electrostatic latent image as a toner image to obtain an image. The optical scanning device of any of the arrangements described above may be used as the optical scanning device for optically scanning the photosensitive member.
An electrostatic latent image is formed by uniform charging of the photosensitive face of the photosensitive member and optical scanning by the optical scanning device. The electrostatic image is developed as a toner image. In the event that the photo-electroconducting photosensitive member is, for example, a sheet-shaped base with a photo-electroconducting layer of zinc oxide or the like formed thereupon, the toner image is fixed to the photosensitive member. In the event of using a photosensitive member such as a Se photosensitive member, the obtained toner image can be transferred and fixed onto a sheet-shaped recording medium such as a transfer paper, an OHP sheet (i.e., a plastic sheet for use with an overhead projector), and so forth.
The image forming apparatus according to the present invention can be carried out as a digital photocopier, an optical printer, an optical plotter, a facsimile apparatus, and so forth.
Note that the number n of partial scanning portions is two or more, as described above, and there is no particular restriction to the upper limit thereof. The number n can be selected as appropriate according to the size and so forth of the area to be scanned.